Attempts have been made to improve a train speed and loading as one of the means for accomplishing higher efficiency of railway transportation. Such an improvement in efficiency of railway transportation means severe use of the rails, and a further improvement in the rail materials has become necessary. More concretely, wear drastically increases in the rails laid down in a curved zone of a heavy load railway and produces a problem from the aspect of longer service life of the rails.
However, high strength (high hardness) rails using eutectoid carbon steels and exhibiting a fine pearlite structure have been developed due to the recent improvements in high-strength rail heat-treatment technology as described below, and rail life in the curved zones in the heavy load railway has been remarkably improved.
(1) Heat-treated rails for ultra-heavy load having a sorbite structure of a fine pearlite structure at the head portion thereof (Japanese Examined Patent Publication (Kokoku) No. 54-25490);
(2) Production method for low alloy heat-treated rails which improves not only the wear resistance but also the drop of hardness at a weld portion by adding an alloy such as Cr, Nb, etc. (Japanese Examined Patent Publication (Kokoku) No. 59-19173); and
(3) Production method for a high strength rail of at least 130 kgf/mm2 produced by conducting accelerated cooling between 850° C. to 500° C. at a rate of 1° to 4° C./sec after rolling is completed or from a re-heated austenite zone temperature.
The characterizing feature of these rails is that they are high strength (high hardness) rails exhibiting the fine pearlite structure of eutectoid carbon-containing steel, and the rails are directed to improve the wear resistance.
In recent heavy load railways, however, an improvement in an axial load of cargos (the increase of train loading) has been strongly promoted so as to further improve railway transportation efficiency. In the case where the rails are sharply curved, the wear resistance cannot be secured even when the rails developed as described above are used, and the drop of rail life due to the wear has become a serious problem. With such a background, the development of rails having a higher wear resistance than that of the existing eutectoid carbon steels has been required.
The contact state between the wheel and the rail is complicated. Particularly, the contact state of the wheels is very difficult at the inner track rail compared to the outer track rail of the curved zone. On the outer track rail of the sharply curved zone of the heavy load railway, for example, the wheel flange is strongly pushed to the gage corner portion by the centrifugal force and receives sliding contact. On the head top portion of the inner track rail of the curved zone, on the other hand, the rail receives great slipping contact having large contact surface pressure from the wheel. As a result, in the case of the high strength wear-resistance rails according to the prior art wherein the head surface hardness is uniform inside the cross-section of the rail head portion, wear is promoted far more at the gage corner portion which receives the sliding contact of the outer track rail that the head top portion which receives the slipping contact of the inner track rail. On the other hand, the progress of the wear is always slower at the head top portion of the inner track rail than at the gage corner portion, and the contact surface pressure from the wheel is always maximal. Therefore, fatigue damage builds up on the head top surface before it is worn out.
The contact state with the wheels tends to the state described above in the high strength wear-resistant rails having uniform wear characteristics at the rail head portion according to the prior art, particularly on the inner track rail of the curve zone. Therefore, if fitting of the rail to the wheel is not quick at the initial wear state immediately after the laying of the rail, a local and excessive contact surface pressure consecutively acts on the rail and a surface damage due to fatigue is likely to occur. In addition, even after fitting is established between the rail and the wheel, a large contact surface pressure always acts on the head top portion and consequently, surface damage similar to so-called “head check”, which generally occurs at the gage corner portion, develops with plastic deformation because the wear is less.
To cope with this problem, there is a method which cuts off the surface layer of the rail head top portion before the rolling fatigue layer is built up. Because the cutting work requires a long time and is expensive, the following rail has been developed.
(4) A high strength and damage-resistant rail exhibiting the fine pearlite structure of eutectoid carbon-containing steel wherein a difference of hardness is provided so that the hardness of the gage corner portion is higher than that of the top head portion in the sectional hardness distribution of the rail head portion, in order to secure the wear resistance equal to that of the conventional high strength wear-resistant rails having a uniform head surface hardness in the cross-section at the gage corner portion, and to reduce the maximum surface pressure (to increase the contact area) by reducing the hardness at the head portion and to improve the surface damage resistance due to the wear promotion action (Japanese Unexamined Patent Publication (Kokai) No. 6-17193).
However, higher axial load of cargos (increase of railway loading) has been vigorously promoted in recent years so as to attain higher efficiency of railway transportation, and even when the rails developed as described above are used, sufficient wear resistance cannot be secured at the gage corner portions of the outer track rail even though they can prevent the surface damage by the periodically grinding of the head top portion in the inner track rail at the sharply curved zone, and the drop of rail life due to wear has been a serious problem.